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Tetrahedron Letters, Vol. 36, No. 2, pp. 217-220, 1995 Elsevier Science Ltd

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Simultaneous Coordination of 2, 6-Dimethylpyranone by 1, 8- Naphthalenediylbis(dichloroborane)

Michael Reilly and Taeboem Oh*

Department of Chemistry, Slate University of New York, Binghamton, NY 13902-6000

Abstract: We show evidence of simultaneous coordination of 2, 6-dimethylpyranone by 1, 8- naphthalenediylbis(dichloroborane) in solution.

Lewis acid complexation to carbonyl compounds is known to have a dramatic effect on reactivity and

selectivity in a variety of reactions. 1 One recent development has been in the area of asymmetric catalysis by

Lewis acids. 2 As part of this development, Lewis acid-carbonyl group interactions have been under continuing

investigation. Simultaneous coordination of a carbonyl group by two Lewis acids is a potential way to increase

the organizational role played by the Lewis acid as well as doubly activate the substrate. 3,4 In a recent

investigation, Wuest has shown that in an intramolecular case, where the carbonyl group was tethered by two

aluminum Lewis acids, both Lewis acids simultaneously coordinated to the central ketone. 5,6 Hine has

reported that the two hydroxyl groups of biphenylenediol can simultaneously hydrogen bond to a carbonyl

group. 7 Kelly has also shown that biphenylenediol promotes Diels-Alder reactions. 8 Our investigation into

this area is two fold. One approach consists of a bimetallic chiral Lewis acid which can simultaneously

coordinate to a carbonyl group (Structure 1). The second is that each Lewis acid plays an organizational role in

coordinating a chiral ligand and a carbonyl group (Structure 2). The subject of this paper involves the

investigation of the equilibrium as shown in equation 1 and to show that structures of the type 1 and 2 are

feasible.

"'0"" : "" "o 3 7 R~'~R L R~R CH H a

1 2 3 4

"o • " "o" (1 )

R~R R~R 5 6

For th is i nves t iga t ion , we have chosen ] , 8 -naph tha lened i y i b i s (d i ch lo robo rane ) 9, 3, and 2,6-

dimethylpyranone, 4. These compounds were chosen for their simple IH NMR spectra and their availability.

There is an added advantage in that the ether oxygen of the pyranone increases the basicity of the carbonyl

group. The IH NMR spectrum of a 2:1 mixture of pyranone 4 and BCI3 show four sharp distinct signals

indicating a slow equilibrium between complexed (8 7.40 and 2.72 ppm) and non-complexed (8 6.20 and 2.29

ppm) pyranone at room temperature. The same was true for a 2:1 mixture of the pyranone and phenylboron

dichloride (complexed, ~i 7.32, 2.70 ppm; non-complexed, ~ 6.38 and 2.35 ppm). Next, a titration experiment

was carried out with 3 and 4 and monitored by 1H NMR (Figure 1). With two equivalents of ketone 4, there

is a downfield chemical shift of 0.64 ppm for H-3 and 0.13 ppm for H-7 (resonances a and c, figure lb).

217

218

a)

b)

c)

d)

CHCI3

I

d

Lj -

b a

i

b

a

d

J~h

8.00 7.50 7.00 15.50 6.00 2.50

Figure 1.1H NMR spectra of 2,6-dimethylpyranone-Lewis acid complex (CDCI3-CD2CI2, 0.05 M in Lewis

acid), a) 4, b) 2:1, 4:3, c)1.5:1, 4:3, d) 1:1, 4:3, e) 1:2, 4:3.

219

There is also small set of signals arising at 7.39 and 2.70 ppm (resonances b and d). As the ratio of Lewis acid

3 is increased, the signal intensities of a and c weaken until they are completely suppressed while the signal

intensifies of b and d increase to completely replace resonances a and c. The resonances a and c are broad and

the aromatic signals of 3 are complex (Figure lb). The signals b and d are sharp and the corresponding

aromatic signals are simple (Figure le). The spectrum ld shows that there is no resonance corresponding to

free pyranone 4.

Spectrum lb indicates that all of ketone 4 is coordinated. This suggests a complex of type 7 where two

ketones are interacting with 3. Complex 7 has several possible conformers involving each donor-acceptor

bond which should result in signal broadening and complex aromatic signals. Complex 8 is a highly rigid

system which should result in sharp signals for the ketone and the naphthalene protons and this is what has

been observed (Figure le). The signals do not have sharp titration points. With two equivalents of ketone 4,

there is still a small amount of complex 8 (Figure lb). With one equivalent of 4, there is a significant amount

of complex 7 (Figure ld), however, with excess Lewis acid 3 the signal due to complex 7 is totally supressed.

• , o " o'"

CH Ha CH Ha tl !

CHa '~ 'O"~ 'CH3 CH3"~ ~O~ "CHa CH H a

7 8 9 10 1 l

Low temperature IH NMR shows that the signals from complex 8 show no chemical shift changes

indicating a highly rigid system. The signals from complex 7, drifted and became sharper. A new signal arose

at 2.75 ppm.

In an effort to further substantiate the existence of the structures 7, 8, 9, and l 0 , a comparison of the

shifts of the characteristic stretching frequencies of the carbonyl and the carbon-carbon double bond was

undertaken using FT-IR. The carbonyl stretch of the coordinated pyranone 7, 8, 9, and 10 show a large shift

(Table I). This shift indicates weakening of the carbon-oxygen double bond. t0 This is presumably caused by

resonance hybrid 12 (Equation 2). Complex 8, where both nonbonding lone pairs are coordinated, shows the

largest shift and complex 7 shows the least shift. The two C=O and C--C stretches of complex 7 are indicative

of the two unequivalent complexes of pyranone. Comparisons of IH NMR shifts show similar trends (Table

II).l 1 From the shift of the carbonyl stretch and the chemical shift, the Lewis acidity of 3, when one boron

atom coordinates to ketone 4, is weaker than PhBCI2. When the two boron atoms coordinate to the ketone

simultaneously the Lewis acidity is comparable to BC13.

The 13C resonance of the carbonyl carbon of pyranone, complex 7, and complex $ were observed at ,BRa

O'" O jBR3

÷

12

(2)

220

TABLE I. IR absorvtion of C---O and C=C (cm:-t) TABLE II. Chemical Shift Comnarisons Entry Comtflex C=O C=C C=C Entry Comolex H-2(~) H-7(8)

1 6 1664.5 1613.7 1597,0 1 6 6.05 2.24

2 7 1498.0 1645.9 1558,0 2 7 6.83 2.42

1495.8 1633.7 3 8 7.43 2.66

3 8 1492.8 1646.9 1552.7 4 9 7.40 2.72

4 9 1499.2 1648.0 1555.8 ~ 10 7.34 2,70

5 10 1~06.0 1650.3 1557.7

180.2, 180.4, and 178.0 ppm respectively. There is a 0.2 ppm shift downfield for the carbonyl peak of

complex 7. The 2.2 ppm upfield shift of complex 8, again, is indicative of resonane hybrid 11. It should be

noted that the 178.0 ppm indicates that complex 8 exhibits a carbonyl moiety. These chemical shifts are

consistent with the literature value in a bidentate coordination of an intramolecular system. 5

As noted earlier, a second equivalent of pyranone prevents the formation of complex 8. Addition of

cyclopentenone, a weaker Lewis base than pyranone, also prevents the formation of complex 8. The 1H NMR

chemical shift of cyclopentenone and pyranone indicates that each are coordinated to one boron Lewis acid

(Structure 11). However, Lewis bases weaker than cyclopentenone do not prevent the formation of 8. For

example, addition of one equivalent of tetrahydrofuran shows complex 8 still exists and there was no

indication of coordination to tetrahydrofuran.

The work presented here is consistent with simultaneous coordination of a carbonyl group with a boron

Lewis acid. There are no sharp titration points and the fact that the equilibrium can be manipulated between

complex 7 and 8 show that simultaneous coordination of carbonyl groups can be enthaipically feasible. The 1,

8-Naphthalenediylbis(dichloroborane) can also coordinate to two different Lewis bases. This behavior has

several implications including the potential use of these type of Lewis acids to play a role in organizing two

substrates.

REFERENCES AND NOTES 1. For review of this area, see: Shambayati, S.; Schreiber, S. L. In Comprehensive Organic Synthesis:

Selectivity, Strategy & Efficiency in Modern Organic Chemistry", B. M. Trost, I. Fleming and L. A. Paquette, Eds., Pergamon Press, New York, 1991, Vol 1, Chapter 1.10, p.283-324.

2. For reviews, see: a) Kagan, H. B.; Riant, O. Chem. Rev. 1992, 92, 1007-1019. b) Narasaka, K. Synthesis 1991, 1-11. c) Oh, T.; Reilly, M. Org. Prep. Proced. Int., 1994, 26, 129-158.

3. Beauchamp, A. L.; Olivier, M. J.; Wuest, J. D.; Zacharie, B. J. Am. Chem. Soc. 1986, 108, 73-77. 4. For a theoretical study of formaldehyde complexed to two equivalents of borane, see: Lepage, T. J.;

Wiberg, K. B. J. Am. Chem. Soc. 1988, 110, 6642-6650. 5. Sharma, V.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1992, 114, 7931-7933. 6. For related complexes, see: a) Adams, R. D.; Chen, G.; Chen, L.; Wu, W.; Yin, J. J. Am. Chem. Soc.

1991, 113, 9406-9408. b) Waymouth, R. W.; Potter, K. S.; Schaefer, W. P.; Grubbs, R. H. Organometallics 1990, 9, 2843-2846. c) Erker, G.; Dorf, U.; Czisch, P.; Petersen, J. L. OrganometaUics 1986, 5, 668-676. d) Adams, H.; Bailey, N. A.; Gauntlett, J. T.; Winter, M. J. J. Chem. Soc., Chem. Commun. 1984, 1360-1361.

7. a) Hine, J.; Anh, K. J. Org. Chem. 1987, 52, 2083-2086. b) Hine, J.; Anh, K. J. Org. Chem. 1987, 52, 2089-2091.

8. Kelly, T. R.; Meghani, P.; Ekkundi, V. Tetrahedron Lett. 1990, 31, 3381-3384. 9. Katz, H. E. Organometallics 1987, 6, 1134-1136. b) Katz, H. E. J. Org. Chem. 1985, 50, 2575-2576. 10. a) reference 5. b) Vedejs, E.; Erdman, D. E.; Powell, D. R. J. Org. Chem. 1993,58, 2840-2845. 11. a) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, 60, 801-808. b) Laszlo, P.;

Teston, M. J. Am. Chem. Soc. 1990, 112, 8750-8754.

(Received in USA 4 October 1994; revised 2 November 1994; accepted 10 November 1994)